1. Field of the Invention
This invention relates to an image data processing method and apparatus, and more particularly to an image data processing method and apparatus wherein non-uniformities of the brightness or the like which arise from some defect of an alignment film of an LCD (Liquid Crystal Display) panel can be detected with a high degree of accuracy.
2. Description of the Related Art
For example, in an inspection process in production of an LCD panel and so forth, it is required to detect non-uniformities of the brightness included in an image data with a high degree of accuracy.
While an LCD panel is produced applying a semiconductor device manufacturing technology such as a manufacturing technique for an integrated circuit (IC), various defects are caused by some non-conformities in the manufacturing process. Because of those defects, when an entirely uniformly white picture is displayed on the LCD panel, the displayed picture suffers from non-uniformities of the brightness (called "brightness non-uniformities" or "area defects"). The brightness non-uniformities include a brightness non-uniformity which is brighter than the surroundings and another brightness non-uniformity which is darker than the surroundings. Those brightness non-uniformities are roughly classified as white detect and black defect, respectively.
An LCD panel includes a liquid crystal layer enclosed between a pair of transparent substrates for which glass substrates or some other suitable substrates are used. In the LCD panel, orientation must be provided to liquid crystal molecules in the liquid crystal layer. To this end, an alignment film is formed on each of the glass substrates such that it contacts with the liquid crystal layer, and in a manufacturing process of the LCD panel, the alignment films are rubbed in a fixed direction using cloth or some other suitable article. The processing of rubbing alignment films in a fixed direction is called rubbing treatment. By performing the rubbing treatment, molecules of the liquid crystal enclosed between the two glass substrates are arranged with longitudinal axes thereof directed in the direction in which the alignment films are rubbed. While an LCD panel is generally formed as a rectangular plate-like member, the direction in which the alignment films are rubbed in the rubbing treatment is set to an oblique direction to a side of the rectangle, for example, in a direction of 45 degrees with respect to a side.
Since an LCD panel exhibits a non-uniform transmission amount of light if the arrangement of liquid crystal molecules is non-uniform, the rubbing treatment is a very important step in production of an LCD panel. However, the alignment films are sometimes rubbed non-uniformly in a striped pattern by some cause. As a result, when an entirely uniformly white picture is displayed on the LCD panel produced, striped non-uniformities of the brightness are produced on the displayed picture. Those brightness non-uniformities may be called "rubbing stripes".
After an LCD panel is manufactured, a picture quality inspection for evaluating such various brightness non-uniformities as described above is essentially required. Conventionally, a test picture is displayed on the LCD panel, and the screen of it is visually observed by an inspector to inspect the picture quality. However, the visual inspection is disadvantageous in that an inspection criterion is indefinite and a dispersion in inspection result is caused by a difference between individual inspectors or by fatigue of the inspector. Therefore, in order to automate the visual inspection, a picture quality inspection apparatus which performs a picture quality inspection based on a digital image data processing technique has been developed. Thanks to development of the image data processing technique, a picture quality inspection apparatus of the type mentioned can even detect brightness non-uniformities which are low in contrast in brightness.
In the picture quality inspection apparatus, an LCD panel on which a test picture is displayed is imaged using a CCD (Charge Coupled Device) image sensor of a high resolution, and various kinds of image data processing are performed using a multiple-valued digital image data obtained by the imaging as an original image data to detect brightness non-uniformities. Even brightness non-uniformities whose contrast in brightness is approximately 3 to 7% can be detected by utilization of filtering for noise removal or contrast emphasis and threshold value processing. An outline of such detection processing of brightness non-uniformities proposed already in Japanese Patent Application No. Heisei 8-28138 by the present inventor is described below. Here, an original image data is a multiple-valued digital image data which is an object of a series of image data processing.
The original image data obtained from an LCD panel includes shading of the brightness arising from a visibility angle characteristic of liquid crystal or some other cause, and the amount of variation of the shading of the brightness is generally larger than the contrast of brightness non-uniformities. Therefore, the shading of the brightness is first removed from the original image data. A mean value filter or a median filter is applied to the original image data to produce a smoothed image data, and the smoothed image data is subtracted from the original image data to remove the shading of the brightness. Here, the smoothed image data is called a shading image data, and the image data from which the shading has been removed is called a difference image data. The difference image data includes, in addition to the brightness non-uniformities, noises including spike-like noises which were intermixed upon imaging and shading components which remain without having been removed. Therefore, the brightness non-uniformities are detected using threshold value processing after a filter for removing the spike-like noises and processing for emphasizing the contrast of the brightness non-uniformities are performed for the difference image data.
A multiple-valued digital image is a two-dimensional array of a plurality of pixels and holds, for each pixel, a value of the pixel as a digital value. Here, each pixel value is an amount corresponding to the brightness of the pixel in a picture having a single color component or a black-and-white picture. The pixel value is represented, for example, by a digital value of 8 bits so that it represents one of 256 gradations. Further, addition or subtraction between two image data signifies to perform addition or subtraction between pixel values of corresponding pixels of the two image data repetitively for the individual pixels to obtain a new image data. Meanwhile, filtering processing is processing of calculating, for each pixel, a sum of pixel values of a plurality of pixels in the proximity of the objective pixel in an inputted image data to determine pixel values of an output image data, and contents of weighting to the pixels in the proximity are different depending upon the contents of the filtering. The weighting coefficients to the pixels in the proximity are normally represented as filter coefficients in the form of a matrix.
However, rubbing stripes are brightness non-uniformities of a very low contrast which are produced from a very small difference in force applied upon rubbing of the alignment films, and according to an experiment, the contrast is approximately 2 to 3%. Therefore, even if the picture inspection apparatus described above is used, it is not easy to detect rubbing stripes. Since the contrast of rubbing stripes is substantially equal to that of noises, if the threshold value is set comparatively low, then also noises are detected, and consequently, it is impossible to selectively detect only the rubbing stripes.